Local CPM ads vs GDN

Hello, I have a question why local ad agencies propose CPM rates are 5 times higher than GDN banners? Any reason behind? Ad agencies can't guaranty conversions and even clicks, all wrong estimations finishing with blia blia..

Local CPM ads vs GDN

If the agency is placing banner ads directly with a publisher, such as a local news affiliate, the CPMs will be much, much higher than on GDN. If the agency is placing ads on GDN themselves and reselling, they typically add in a percentage for management.